“It is indisputable that although we can’t ascribe any particular kind of weather event to rising temperatures, that at the current pace that we are on, the scale of tragedy that will consume humanity is something we have not seen in perhaps recorded history if we don’t do something about it,” Obama said during a trip to Canada, according to CTV News.

“Imagine when you have not a few hundred thousand migrants who are escaping poverty or violence or disease, but you now have millions. Imagine if you start seeing monsoon patterns in the Indian subcontinent changing so that half a billion people can’t grow food and are displaced,” he added. “Think about what that does to the politics of the world — not just the economics of it, not just the environment.”

Obama was an outspoken proponent of combating climate change while in the White House. Under his leadership, the U.S. was one of the signatories of the historic Paris climate accord, in which world powers agreed to take measurable steps to reduce their man-made footprint on the environment. President Trump announced in 2017 that the U.S. would withdraw from the agreement.

The former Democratic president on Tuesday told a packed crowd in Calgary, Alberta, home to many struggling oil and gas companies, that nobody can escape the impacts of climate change.

“All of us are going to have to recognize that there are trade-offs involved with how we live, how our economy is structured, and the world that we’re going to be passing on to our kids and grandkids. Nobody is exempt from that conversation,” Obama said.

He also noted that rising oceans risk coastal populations and environmental changes have boosted the frequency of insect-borne diseases.

“Moose right now [have] to deal with tick-borne diseases that they didn’t have to do 10, 15 years ago. I really like moose. I assume, Canadians, you do too,” Obama said. “These are just facts.”